Bonded abrasive articles



y 1951 K. H. SANDMEYER 55 BONDED ABRASIVE ARTICLES Filed Feb. 21, 1958INVENTOR. AAEL H. $ANDM)Z-'E A r roe/v5 United? States l stens BONDEDAsaAsrvn ARTICLES Karl H. .Sandmeyer, Jamestown, N.Y., assignor to TheCarborundum Company, Niagara Falls, 'N.Y., a corporation of DelawareFiled Feb. 21, 1958, Ser. No. 716,641

8 Claims. (Cl. 51296) This invention relates to bonded abrasive articlesof manufacture such as grinding wheels and other shapes comprising abody or mass of particulate abrasive material held together in aninterstitial bonding matrix.

Grinding Wheels and other bonded abrasive shapes must not only contain aparticulate abrasive material that is held in a surrounding matrix orbond with sutficient tenacity to stand up under the stresses anddisruptive forces of the grinding operation, but the body structure ofthe article must be such that the cutting performance of the article issatisfactorily efficient from the stand point of rapidity of abradingaction or removal of ma- "meal from the object being ground coupled withan acceptably low rate of wear of the abrasive body. These structuralrequirements for an abrasive body vary considerably with the kind ofmaterial to be ground.

The conventional abrasive wheel of the prior art employs a granularmaterial of selected grit size or sizes such as solid particles orgranules of alumina, silicon carbide, boron carbide, zirconia or thelike obtained by crushing the crude furnaced material in pig or ingotform to the desired particle size or sizes. While the cuttingcharacteristics of the article are determined to some extent by thenature of the particulate abrasive material used in the wheel, and thetype and amount of bonding material used therewith, porosity or opennessof the bonded structure to provide coolness of cutting action andfreedom from loading is customarily obtained by incorporation within thebond of a pore-forming material that is either volatile or combustibleso as to be driven off during the curing or firing of the article toleave a structure of voids within the article when cured or fired, or amaterial is used that remains in the wheel but is weak and friable inthe finished article so as to be readily removed during the grindingoperation.

It is an object of the present invention to provide abrasive wheels andother bonded shapes of satisfactory open body structure without resortto the use of pore-forming materials or fillers.

It is a further object of the invention to provide bonded abrasivearticles especially adapted for the grinding of materials requiring anabrasive body of open structure for the grinding operation.

Other objects and advantages accruing from the present invention willbecome obvious as the description proceeds.

I have discovered that grinding wheels and other bonded abrasive shapesof porous, open structure and freecutting characteristics and especiallyadapted for the cutting and grinding of certain materials requiring suchopen structures in the grinding medium can be made in which part, andpreferably all, the abrasive component of the article is in the form ofhollow spherical or globular abrasive particles. One such sphericalabrasive material that I have found highly satisfactory for use incarrying out the present invention is fused alumina bubbles.

Alumina bubbles are a well known industrial product, having beenoriginally developed for use in the making of aluminum metal byelectrolytic reduction methods, and subsequently used also as arefractory insulating ma- Patented May 30, 1961 terial. Alumina bubblesare available in particle sizes as coarse as 6 mesh and finer. Methodsof making fused alumina bubbles or hollow spheroidal abrasive particlesof other composition are well known and need no further descriptionherein. Briefly, fused alumina bubbles are made by fusing .a high gradeof alumina to the molten condition and pouring a stream of the moltenmaterial onto a rapidly rotating disc or into a high velocity gaseousblast to dissipate the material into fine droplets that solidify to formhollow spherical particles, the size of which depends upon such factorsas the size, cornposition and temperature of the molten stream and thevelocity of the gaseous blast or rotating disc used for dissipating thestream.

In making abrasive articles in accordance with the present invention,the alumina bubbles or other spherical abrasive particles of selectedparticle size or sizes are mixed with a suitable bonding material, carebeing taken to avoid as much as possible any crushing or breakdown ofthe hollow abrasive spheres. The bond can be modified by the inclusiontherewith of inert or active fillers and other modifying agents, as maybe desired. The resulting mix is placed in a mold and subjected topressure to form an abrasive body of the desired shape. The moldedarticle is removed from the mold and subjected to the necessary heattreatment to mature the bond. The times and temperatures required formaturing the bond are those conventionally followed in the making ofbonded abrasive articles and depend upon the type of bond used. Ceramicbonded articles are always matured following the removal from the mold,whereas articles utilizing an organic bond, such as a phenolic resincondensation product, can be either formed by a hot-pressing operationin which the bond is partially or fully matured at the time of formationin the mold by simultaneous exposure to heat and pressure, or thearticle can be molded, removed from the mold, and the bond subsequentlycured by a time-temperature treatment. After curing or firing, theabrasive body is suitably dressed in accordance with conventionalpractices. For example, grinding wheels after maturing of the bond arebored, faced, bushed, edged and speeded in accordance with standardpractices employed in the manufacture of bonded abrasive wheels.

In order that the invention may be more clearly understood, reference ismade to the various figures of the drawing which present illustrativeembodiments of the present invention, and in which:

Figure 1 is a side view of a grinding wheel made in accordance with thepresent invention;

Figure 2 is a highly enlarged schematic sectional view of a fragment ofa resin-bonded grinding wheel made in accordance with the presentinvention; and

Figure 3 is a view similar to Figure 2, depicting a ceramic-bondedgrinding wheel fragment.

The following specific examples are illustrative only and are not to beconstrued as in any way limiting the scope of the present invention.

EXAMPLE 1 Resin bonded alumina bubble grinding wheels of the typedepicted in Figure 1, consisting of a mass or body 5 of resin bondedalumina bubbles and provided with a central mounting arbor 6, were madeas follows:

The alumina bubbles were first wet by mixing with the solvent solution,after which the powdered phenolic resin was added in small incrementswith continued mixing' until all the powdered resin had been taken upand distributed throughout the mixture of alumina bubbles. A phenolicresin that can be used is that known and sold by the Bakelite Co. asBakelite BRP-54l7 phenolic resin. Mixing was carried out in a 12"Lancaster mixer using mixes up to 3500 grams in size. Approximately 2minutes mixing time was required for wetting the bubbles with thesolvent solution followed by approximately 4 to 6 minutes mixing timefor adding and mixing in the powdered resin. Because of the coarse sizeof the alumina bubbles, screening of the mix was not required.

Best results were obtained when the mixes were used for molding withinone or two hours after preparation of the mix. The resulting mix wasplaced in the mold cavity and pressed at a pressure of 1,000 pounds persquare inch with a holding time of approximately secends. The resultingwheel was removed from the mold and subjected to the following ovencuring schedule. The temperature was raised from room temperature to 350F. in 20-30 hours, held at 350 F. for 5-10 hours, and

cooled gradually down to room temperature over a period of -20 hours.

An alumina bubble grinding wheel, 12 inches outside diameter, 1 inchthick and 5 inches inside diameter or arbor, made in accordance with thecomposition and procedure of Example 1 above, had a density of 23.0grams per cubic inch in the uncured condition and 22.4 grams per cubicinch density after cure. Another grinding wheel made according toExample 1 above and having an outside diameter of 8 inches, thickness ofof an inch and a inch arbor, when subjected to a speed test, broke at19,720 surface feet per minute.

Examination under the microscope of the body structure of resin-bondedalumina bubble grinding wheels made in accordance with Example 1 above,and reference in connection thereto is now made to Figure 2 of thedrawing, discloses that the alumina bubbles 7 are held together in aninterstitial matrix of cured resin, a portion 8a of the resin bondingmaterial impregnating crevices and pores in the walls of the individualalumina bubbles to strengthen and reinforce them. Examination of thestructure of the wheel further discloses that porosity or open structurewithin the wheel is provided to a marked extent in the form of cavities9 within the individual alumina bubbles making up the wheel structure,thereby avoiding the need for an extraneous pore-forming material as aconstituent of the wheel body. One particular advantage of obtainingporosity within a wheel structune by means of the use of hollowspheroidal abrasive particles is the fact that the porosity is notderived from pores buried within the bond but is directly associatedwith the abrasive particles so that the latter are fully exposed toprovide optimum cutting performance during use of the abrasive body.

Resin bonded alumina bubble grinding wheels, made in accordance withExample 1 herein, have been used with highly satisfactory results forthe grinding of rubher, paper fiber board materials and plastics.

EXAMPLE 2 Grinding wheels 12 inches outside diameter, 1 inch thick and1%. inches inside diameter were made from The molded shapes had adensity of 26 grams per cubic inch in the uncured condition and adensity in the cured 4; condition of 25.7 grams per cubic inch. Theresulting wheels rendered excellent performance in the grinding ofrubber rolls such as typewriter platens and the like.

EXAMPLE 3 Rubber bonded alumina bubble grinding wheels can be made asfollows:

Parts by weight Alumina bubbles 70-90 Depolymerized rubber 5-20Pulverized sulphur 2.5-10 Powdered phenolic resin 5-15 Mineral filler(e.g., cryolite, MgO, clay) 0-30 The above ingredients are thoroughlymixed by adding the depolymerized rubber to the alumina bubbles to coatthem, after which the dry ingredients are mixed in thoroughly, neutralcreosote oil to the extent of 5-20 parts by weight per 1000 parts of drymaterials being added as needed to maintain bond dispersion. Theresulting mix is placed in a mold and hot-pressed at 300 F. and 500-2500p.s.i. to vulcanize the rubber, the wheel being held in the mold 5-20hours depending upon the size of the wheel.

EXAMPLE 4 Ceramic bonded grinding wheels, size 8 inches outside diameterby /2 inch thickness by 2 inches inside diameter and size 12" x 3" x1%", were made from the following The above ingredients were intimatelymixed in a 10" Hobart mixer. The alumina bubbles were first wet with thewater after which the ceramic bond and dextrin were blended in andmixing continued until the bond uniformly coated the alumina bubbles.Wetting of the alumina bubbles required approximately 2 minutes mixingtime and blending in of the ceramic bond and dextrin required anadditional 4 to 6 minutes mixing. The mixes were not screened. Theresulting mix was placed in a steel mold and pressed at 1000 pounds persquare inch with a holding time at maximum pressure of 5 seconds. Themolded shapes were removed from the molds, dried and fired at 1250 C.The time required to reach the maturing temperature (1250 C.) variesfrom 12 to 84 hours, depending upon the size of the wheels and the kilnload. Similarly, holding time at maturing temperature ranges from 4 to12 hours, followed by cooling at a rate somewhat slower than the heat-uprate. The fired wheels were edged, finished and bushed according tostandard practice and speed tested prior to use. The resulting wheelsare useful in the grinding of concrete and bronze.

Figure 3 shows schematically a fragment of a ceramic bonded wheel bodymade according to Example 4, and reveals the alumina bubbles 7 heldtogether in the inter stitial ceramic bond 10. The porosity of the wheelstructure is provided in the form of the cavities or voids 9 within thebubbles 7.

EXAMPLE 5 The following mix was used to make wheels 8" x /2" x 2" insize with satisfactory results, the proce- 5 dure for making the wheelsbeing the same as that set forth above for Example 4. Mix as follows:

Parts by weight The foregoing examples are representative of the presentinvention in its preferred form wherein the alumina bubbles or otherhollow spheroidal abrasive par- .ticles constitute the sole abrasiveconstituent of the abrasive article. However, the invention can also befollowed in modified form by incorporating the hollow spheroidal hard,abrasive particles in the abrasive body in conjunction with conventionalgranular abrasive material wherein the hollow spheroidal abrasivematerial, because of having a hardness comparable to the abrasive gminofthe body, functions as an abrasive and as a filler, and yet introducesthe same type of porosity or openness of structure in the abrasive bodyas that referred to above :in these examples where it constitutes thewhole of the abrasive component of the article, but to a lesser extent.The following examples are illustrative of such modified forms ofabrasive bodies.

Abrasive grain 65-95 Powdered phenolic resin 5-25 Filler (such ascryolite) -15 Alumina bubbles l-30 Solvent solution (75% furfural, 25%cresol),

10-35 parts/1000 parts of dry ingredients The mix is prepared inaccordance with the procedure set forth above for Example 1 and theabrasive Wheels formed by cold-pressing at 500-3000 p.s.i., followed bya soaking bake of 5 to hours at 350 R, with suitable heat-up andcool-down periods similar to those given above for Example 1.

As an example of the making of abrasive bodies in accordance withExample 6, 12" x 1" x 4" phenolic resinbonding grinding wheels embodying24 grit silicon carbide grain, and similar size grinding wheelsembodying 24 grit fused aluminum oxide grain, together with 6% 6-14 meshfused alumina bubbles were made and used on a surface grinder at a tablespeed of 40 feet per minute and an infeed of .005" to grind the ends of1" black iron pipe with highly satisfactory performance results.

EXAMPLE 7 Resin bonded grinding wheels can be made by hotpressingprocedure, using the following compositions:

Parts by weight Abrasive grain 65-80 Powdered phenolic resin (such asBakelite BRP- 5417) 8-15 Fillers, such as cryolite 5-15 Fused aluminabubbles 1-10 Lime 0-3 Solvent solution (75% furfural, cresol), 5-25parts by weight/ 1000v parts dry ingredients.

For example 20" x 2 /2" x 6" phenolic resin-bonded grinding wheels,using fused aluminum oxide abrasive grain, and containing 1.6% by weightof 16-40 mesh size alumina bubbles, were made by hot-pressing for onehour at 3000 psi. and 330 F., followed by a soaking bake at 325 F. for10-20 hours. The resulting wheels performed satisfactorily in thegrinding of Type 304 stainless steel at 230 pounds work pressure.

EXAMPLE 8 Vitrified bonded grinding wheels were made as follows:

Mix A Parts by weight 20 grit fused alumina s e 900 Ceramic bond (sameas Example 4) Dextrin 20 Water -7 V 20 Mix B 14 .and finer aluminabubbles 770 Ceramic bond .(same as Example 4) 23.0

Dextrin 30 Water 37 The above compositions A and B were first mixedseparately and then intimately blended. The resulting blended mixturewas placed in a mold and an 8" x /s" x 1 grinding wheel formed bypressing at l000-p.s.i., and tired at 1250" 10. as described in Example4. After finishing, the wheel was used for grinding bronze and similarsoft metals with satisfactory performance results. Wheels so made weretested for breaking strength and broke at speeds of 17,794 s.f.p.m.

The specific examples set forth above are not to be considered to :berestrictive of the invention. For example, other types and sizes ofabrasive wheels, including cut-01f wheels, and other abrasive shapes,can be made in accordance with the practices of the present invention.Although all the specific examples have dealt with the making ofarticles wherein the desired shape has been made by placing themoistened mix in a mold and subjecting the mold contents to pressure,and the bonded article is matured by heat treatment, other formingpractices common in the abrasive art can be followed without departingfrom the present invention such as preparation of a mix of suitableconsistency for casting and forming the desired shape by pouring the mixinto a mold and allowing it to set or harden. Also, in the case ofsilicate or glue bonded articles, the shape can be matured by eitherair-drying or a low temperature oven treatment of the article to set orharden the bonding constituent.

Also, while alumina bubbles have been used in the specific examples asthe abrasive material, other hollow spherical abrasive material such ashollow fused zirconia abrasive spheres can be used. It is not intendedto include or embrace within the scope of the present invention the useas a filler material of such non-abrasive substances as fine, porous orglobular clay, resinous of vitreous articles.

Furthermore, the invention is not deemed to be restricted to anyspecific organic or ceramic bonding material, or to any specific amountof bond, although abrasive bodies of the present invention normallycontain more than 50% by weight of the abrasive constituent orconstituents.

Various inorganic or ceramic bond compositions can be used, both of theporcelanic type or vitrified type, such as those derived from frits,clays and frit-clay blends, silicate such as sodium silicate bonds, andmagnesium oxychloride cement bonds.

Other organic bonding materials in conjunction with conventional fillersand other modifying agents can be used in carrying out the presentinvention, including urea formaldehyde resins, melamine formaldehyderesins, epoxy resins (bis phenol A-epichlorohydrin), polyester resins,alkyd resins, shellac, glues, vulcanized rubber derived fromdepolymerized (melted) crude or butadieneacrylonitrile rubbers, naturalor synthetic rubber latices and the like.

Actual grinding operations with abrasive wheels made in accordance withthe present invention demonstrate that the resin-bonded wheels areparticularly suitable for the grinding of rubber, leather, wood,plastics, bonded particularly useful in the grinding of concrete andbronze.

Having described the invention in detail, it is desired to claim:

1. An abrasive article comprising spheroidal, fused particles of aluminaheld together in an interstitial resinoid bonding matrix, said particlesbeing hollow thinwalled bubbles each having a single central cavity.

2. An abrasive article comprising spheroidal, fused particles of aluminaheld together in an interstitial phenolic resinous condensation productbonding matrix, each said particle being a hollow thin-walled bubblehaving a single central cavity.

3. Abrasive article adapted for the grinding of rubber, leather, woodand plastic materials, said article comprising a body of smallspheroidal fused abrasive particles held together by a resinous bond,each said abrasive particle being a hollow thin-walled bubble having asingle central cavity.

4. An abrasive article adapted for the grinding of rubber, leather, woodand plastic materials, said article eomprising a body of smallspheroidal fused abrasive particles held together by a resinous bondsaid abrasive particles being hollow thin-walled bubbles each having asingle central cavity, a portion of said bond penetrating andimpregnating the walls of the abrasive particles to strengthen them. i

5. An abrasive article according to claim 4 in which the spheroidalabrasive particles are fused alumina bubbles.

6. An abrasive article according to claim 4- in which the bond is aphenolic resinous condensation product.

7. A bonded abrasive article comprising hollow spheroidal, fusedabrasive particles selected from the group consisting of fused aluminabubbles and fused zirconia bubbles, each said bubble having a single,central eavity, said bubbles being thin-walled and being held togetherin an interstitial bonding matrix selected from the group consisting ofsilicate of soda, shellac, rubber, and a resinoid.

8. A bonded abrasive article comprising a plurality of hollow, generallyspherical, thin-walled bubbles of fused abrasive material selected fromthe group consisting of fused alumina and fused zirconia, each saidbubble having a single, central cavity, and an interstitial matrixbonding said particles together and selected from the group consistingof silicate of soda, shellac, rubber, and a resinoid.

References Cited in the file of this patent UNITED STATES PATENTS1,133,739 Morse Mar. 30, 1915 1,192,709 Tone- July 25, 1916 1,263,709Saunders et a1. Apr. 23, 1918 1,524,134 Hutchins Ian. 27, 1925 1,956,905Merriam May 1, 1934

8. A BONDED ABRASIVE ARTICLE COMPRISING A PLURALITY OF HOLLOW, GENERALLYSPHERICAL, THIN-WALLED BUBBLES OF FUSED ABRASIVE MATERIAL SELECTED FROMTHE GROUP CONSISTING OF FUSED ALUMINA AND FUSED ZIROCONA, EACH SAIDBUBBLE HAVING A SINGLE, CENTRAL ACTIVITY, AND AN INDUSTRIAL MATRIXBONDING SAID PARTICLES TOGETHER AND SELECTED FROM THE